1. Field of the Invention
The present invention relates to an improved method of calculating an internal resistance of a secondary battery (or a storage battery) mounted on a motor vehicle.
2. Description of the Related Art
An internal resistance as one of internal impedances of a secondary battery (or a storage battery) mounted on a motor vehicle is a well-known important parameter in order to judge a deterioration state of and a charge state of the secondary battery, For example, using an internal resistance as an input parameter into a Neural network simulator capable of performing a state of charge (SOC) calculation can detect a SOC of the secondary battery with high accuracy.
Related art techniques calculate such an internal resistance of a secondary battery based on an inclination of a regression line obtained using a current-voltage (I-V) characteristic of the secondary battery. For example, Japanese patent laid open publication No. JP 2002-343444 and No. JP 2005-146939 have disclosed such a method of calculating an internal resistance of a secondary battery based on an inclination of a regression line.
However, because the I-V characteristic of a secondary battery such as a lead storage battery is much changed and a polarization state of the secondary battery is thereby largely varied, it is difficult to obtain the internal resistance based on the I-V characteristic of the secondary battery for a motor vehicle with high accuracy. In order to solve such a drawback, there is another related art technique for calculating an internal resistance of a secondary battery with high accuracy using an I-V characteristic measured at an engine start where a change of a large current occurs in a short period of time. This method is referred to as the “internal resistance calculation method at an engine start”.
Such an internal resistance calculation method when a vehicle engine starts is requested to measure a I-V characteristic within a large current range in order to increase the calculation accuracy for the internal resistance of the storage battery.
The internal resistance calculation method further requires a current sensor capable of measuring a large currant within a wide current range with desired high accuracy in order to measure a starting current when a vehicle engine starts within a full detection range such as from −1000 A to −600 A. The current sensor having such a capability is extremely expensive because it guarantees a desired accuracy at any detection point in the full detection range.
Still further, the current sensor having such a function detectable a wide current range is only used when the vehicle engine starts, in other words, is not used in a case other than the vehicle engine start. In addition, because the engine starting current varies from a minimum value to a maximum value for a short period of time, for example, within a short-time range of 1 to 3 msec, it is in general difficult to perform samplings of the engine starting current in a wide range at a high speed with high accuracy.
In order to solve such a related art problem in views of necessity and economical efficiency, the related arts use a current sensor (or a middle range current sensor) capable of measuring a current within a range of −500 A to +100 A in order to measure an engine cranking current. FIG. 11 shows a current waveform when a vehicle engine starts detected by such a middle range current sensor. As clearly understood from FIG. 11, the middle range current sensor cannot detect a discharge current in a wide current range when a vehicle engine starts.
However, the use of the current-voltage characteristic during a cranking detected by such a middle range current sensor causes a difficulty in detecting an internal resistance of a secondary battery (or a storage battery) with high accuracy because a polarization state of the secondary battery largely affects the detection accuracy of the internal resistance of the secondary battery, as shown in FIG. 12.